The assignment of elevator car calls as soon as they are registered, so as to permit persons to queue in front of the hoistway door of the car which is expected to answer the call, and to provide reassurance to passengers, is typically made in response to predictions. In commonly owned copending U.S. Pat. No. 5,427,206, assignment of hall calls is based upon the car which is predicted to get there most quickly, unless it causes other calls to become "elderly"(or more so); the term "elderly" meaning that it has been predicted that the call would not be answered in a minute or less. The problem with the system of the aforementioned application is that even though a car could answer the call in question extremely quickly (for instance, in less than 10 seconds), if such assignment would cause the predicted response to any other call to advance from 59 to 60 seconds, or from 61 to 62 seconds, thereby either causing it to become elderly or more elderly, that car would not get the assignment; this is true even if all of the remaining assignments might take 40 or more seconds and would cause calls to have to wait 57 or 59 seconds. In such a circumstance, obviously the first car would be a better assignment than any of the others, but such an assignment would not be made. Better hall call assignments are provided in the method of a commonly owned copending U.S. patent application entitled "Elevator Dispatching Employing Hall Call Assignments Based on Fuzzy Response Time Logic" U.S. Ser. No. 08/264,842, filed contemporaneously herewith. However, when hall call assignments are made early in the life of the call, there is significant opportunity for delaying the assigned car as it proceeds through a variety of service events toward the call. Such delays may commonly be caused by an unusually large number of exiting or entering passengers, holding doors open during conversations, and the like.
In instantaneous car assignment protocols, the theory is that the assignment should never be changed to a different car after the assignment to a particular car is announced, because passengers are required to move to a new car and, in some cultures, become confused. For this reason, many elevator owners insist that no more than some small percent (such as two percent) of elevator calls shall be reassigned. However, if the initial assignment is determined to be truly inferior, and there is a much superior choice of a car to answer the call, then the call should be reassigned. In some cases, it is possible that, due to equipment conditions, the call would never be answered by the assigned car.
It has been known to examine assigned call criteria, and if the predicted waiting time exceeds an "elderly" threshold, such as 45 seconds, and there is another car that could possibly reach the call in a much shorter time, such as ten seconds, then reassignment of the car is made. On the other hand, when a call's PWT is slightly below the threshold, (e.g., PWT=40 seconds), the call will not be considered for reassignment, even though an excellent candidate car exists for reassigning. The problem is that this excellent candidate car may very well have passed right by the call, for instance, some 6 seconds from now when the PWT exceeds the threshold.